It has long been known that a troublesome vibrational phenomenon, often referred to as chatter, tends to arise during metal-turning operations. Undesired displacements of the boring bar, due to chatter, degrade the surface quality of the finished workpiece. Chatter tends to become more prevalent as the boring bar is made longer, and as the depth-of-cut or feedrate is increased. As a result, there is often a tradeoff among surface quality, throughput, and bore length. In particular, the potentiality for chatter severely limits the surface quality that can be obtained, at reasonable cost, in long bores. By way of example, products that are made with such long bores include aircraft engine parts, gun barrels, and high-performance pump parts.
Practitioners in the metal-cutting arts have tried various stratagems for suppressing unwanted mechanical vibrations, including chatter. Passive suppression techniques attempt to dissipate vibrational energy in dense viscous fluids, viscoelastic pads, and the like. In some of these approaches, energy is dissipated when the free motion of one or more massive inertial bodies is opposed by frictional or viscoelastic forces.
Another approach to the suppression of vibrations in a mechanical system involves techniques from the well-known field of active control. In active control, a sensor or array of sensors detects unwanted vibrations and sends information about these vibrations to a signal processor. In response, the signal processor sends driving signals to a mechanical actuator or array of mechanical actuators. The actuator or actuator array applies forces to the mechanical system that are devised to at least partially counteract the unwanted vibrations.
Numerous techniques are known for computing, within the signal processor, appropriate counteracting forces. These techniques include, for example, classical linear feedback techniques, and adaptive feedback techniques using finite impulse response (FIR) or infinite impulse response (IIR) filters.
U.S. patent application Ser. No. 08/640,396, filed by R. Benning et al. on Apr. 30, 1996, describes one such computing technique that is especially useful in the presence of vibrational noise that is periodically regenerated by rotation, such as the rotation of a workpiece on a lathe.
U.S. Pat. No. 5,170,103, which issued to K. E. Rouch et al. on Dec. 8, 1992 (hereinafter, "the Rouch patent"), describes one application of active control methods to the chatter problem. The Rouch patent discloses a motion sensor and a force generator mounted near the free end of a boring bar (or other cantilevered beam-like member). The boring bar has a hollow portion, and the force generator is mounted within this portion. The force generator includes an inertial mass which is fastened via a hinge to the boring bar. The force generator further includes an actuator, and an element such as a spring for applying a restoring force to the inertial mass.
Although it is useful, the device disclosed by the Rouch patent has an inherent limitation. That is, the boring bar to be used with this device must be made large enough in diameter to enclose the force generator. However, the force generator cannot be made arbitrarily small. Therefore, this device cannot be used within bores smaller than a minimum diameter determined by practical limits on the miniaturization of the force generator. Current technology limits the diameter of the enclosing boring bar to about five centimeters or more.
Moreover, current approaches, including that described in the Rouch patent, involve an actuator that is integral with the boring bar. Because ordinary boring bars cannot be used, the practitioner must choose between obtaining many special boring bars at great expense, or working with a limited selection of boring bars.
Thus, until now, active control systems have lacked a device for mechanical actuation that is adaptable for use with multiple boring bars, and in particular, for use with boring bars of arbitrary diameter.